1. Field of the Invention
This invention relates generally to dynamic random access memory (DRAM) units and, more particularly, to the apparatus incorporated in the DRAM units which maintain the voltage levels in the DRAM unit within predetermined tolerances. The maintenance of specified voltage levels, especially the substrate bias voltage level, is especially critical in the DRAM units in order not to compromise the operation.
2. Description of the Related Art
The DRAM unit has found wide use in data processing system memory applications requiring low power, low cost, and high densities of storage cells. System designs using battery powered DRAM units for normal operation and/or for data storage backup, i.e., in notebook computers, space applications, main memory in data processing units equipped with data recovery from power supply failures, etc., are becoming increasingly important. System designs which can provide lower standby power are similarly of increasing interest.
The maintenance of a predetermined substrate bias voltage level within a DRAM unit is typically provided by a charge pump which maintains voltage (and charge) levels within a predetermined operating range during operation of the DRAM unit. Typically, a level detector is provided to detect the passage of the voltage from a level within the operating range of the system to a voltage level outside of the operating range. When the voltage level passes outside of the predetermined operating range, the charge pump is activated to alter the charge on the semiconductor substrate, (i.e., to alter the voltage across the plates of the storage cell capacitor).
Analog techniques, where the operation of the charge pump is controlled by the bias value, have been reported. But these techniques have inherent problems such as leading the chip to a danger level of substrate bias before the charge pump can respond and restore the level.
A need has therefore been felt for apparatus and an associated method for providing apparatus for controlling the substrate bias voltage operating at low power during standby, for providing easy incorporation in DRAM units which are already being fabricated.